1. Technical Field
This invention relates generally to document production apparatus such as copiers, printers, and other marking engines having the need to transport media with high position and velocity control to maximize the registration accuracy of the media, thereby insuring the quality of the document.
2. Background Art
Document production apparatus such as copiers, printers, and other marking engines use a variety of methods for moving media so that images can be transferred onto hard copy output paper or transparency media. For example, color thermal printers use a media transport control system to move media (receiver webs and sheets) beneath a thermal print head.
In color printers, the quality of the final print image is directly related to the registration accuracy of the successive color planes on the final print. Therefore, media transport systems for color printers are designed to very accurately control the position and velocity of the media so as to maximize the ability to print two or more successive color planes in a highly registered fashion.
Accurate position and velocity control is especially difficult in thermal printers because the required media velocities are generally very low relative to other types of document production apparatus. Therefore, designers have long attempted to employ methods for enhancing the position and velocity control of the media. Such methods have ranged from simple to exotic. For example, a simple method might include the use of a stepper motor with a speed reduction transmission system to drive a rotating drum or roller; while an exotic method might include a closed loop feedback system to control a DC motor that powers a drum or roller.
One such closed loop feedback system is shown in FIG. 1, wherein a computer-generated desired-position command is compared with a trigonometric signal (explained below) by a digital comparator 10. The difference signal is converted to analog form at 12 and input to a compensation network 14 for filtering. The filtered signal is amplified at 16 and used to drive a DC motor 18 for positioning the media via, say, a rotating drum or roller. The media position is detected by a digital position sensor 20, which creates the trigonometric signal which was referred to above as one of the inputs to comparator 10.
While motion control loops such as shown in FIG. 1 are widely used, they have the disadvantage of requiring many performance tradeoffs, depending on the designer's choice of subsystems. For example, amplifier 16 may be a voltage drive or a current drive. If a voltage drive amplifier is chosen, inherent speed control is provided by the back electro-motive force of DC motor 18. The system lacks bandwidth due to mechanical and electrical parameters of voltage drive amplifier 16 and DC motor 18. That is, the system has limited positional resolution control related to the number of bits either at digital-to-analog converter 12 or at digital position sensor 20, whichever is lowest. Also, the quantization at digital-to-analog converter 12 creates compensation problems for compensation network 14 of the "feed forward" type.
If, on the other hand, a current drive amplifier is chosen for amplifier 16, the bandwidth for the feedback system can be increased. However, a velocity control feedback sensor or state space estimation should be used to stabilize the loop and provide velocity control. Even with this velocity control, performance will be marginal for high performance image printing. One solution to this poor velocity control is to make sure that compensation network 14 has at least two integrators so as to drive velocity errors to zero for a constant velocity input (position ramp input). However, the ability to stabilize these loops usually becomes extremely difficult and costly. In addition, the resolution and quantization problems mentioned above will still exist.
It has also been suggested to at provide precision velocity control by utilizing a motion control loop such as illustrated in FIG. 2. This particular motion control loop utilizes a phase lock loop for the velocity control portion of the motion control loop. It has many of the same subsystems as the motion control loop of FIG. 1, identified by primed reference numerals, but utilizes a phase detector 24, a switch 26, and a magnitude comparator 28 in place of the digital comparator 10 of FIG. 1.
Basically, this motion control loop of FIG. 2 separates the position and velocity controls. Switch 26 will start in the position mode as illustrated while a desired position profile and feedback from digital position sensor 20' are compared at magnitude comparator 28. When the loop approaches a speed equivalent to a desired constant speed (such as in the printing mode of a printer), the state of switch 26 is changed to allow the velocity mode to take over, whereupon the phase detector 24 is used to provide an error signal for the motion control loop. When in the velocity mode, the motion control loop provides ultra precision velocity control, but does not know where it is in absolute position space (an external counter could keep track of the digital position sensor 20' divisions but cannot provide control). In addition, the position portion of this motion control loop of FIG. 2 still has the limiting capabilities described for the motion control loop of FIG. 1.